Internal combustion engines, and in particular Diesel engines, are known to be equipped with exhaust gas aftertreatment systems. Aftertreatment systems treat exhaust gases that exit the combustion chamber and that are directed into an exhaust pipe having one or more aftertreatment devices configured to filter and/or change the composition of the exhaust gases. For example, aftertreatment devices may include an Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), a Lean NOx Trap (LNT), a Selective Catalytic Reduction (SCR) system, a SCRF (SCR on Filter) or combinations thereof.
Exhaust Gas Recirculation (EGR) is also a known NOx emission control technique applicable to a wide range of diesel engines. The configuration of an EGR system depends on the current legal requirements expressed in terms of required EGR rate vs engine operating area and on EGR system costs and packaging constraints. Advanced EGR systems include a high pressure EGR conduit which fluidly connects the exhaust manifold with the intake manifold, and a low pressure EGR conduit which fluidly connects the exhaust line downstream the DPF to the intake line upstream the intake manifold.
While the high pressure EGR conduit defines a short route for the exhaust gas recirculation, the low pressure EGR conduit defines a long route which also includes a relevant portion of the exhaust line and a relevant portion of the intake line. In this way, the long route EGR (LRE) is effective for routing back to the intake manifold exhaust gas having lower temperature than those routed back by the short route EGR (SRE). The usage of the Low Pressure EGR is currently limited when the exhaust gas temperature downstream of the filter is high, as for example during or soon after a DPF/SCRF regeneration and a LNT desulphation process.
As is known in the art, a LNT desulphation process, also known as a DeSOx regeneration event, is performed by several rich combustion phases executed at high temperature, where gas temperature in the LNT may be around 650° C. Each rich combustion phase being followed by a lean combustion phase, whereby this lean-to-rich-to-lean approach is also referred as wobbling approach. In these conditions the exhaust gas temperature might reach values higher than 400° C. and the recirculation of these exhaust gas stream might cause severe problems to the EGR valve since its structural limit is 180° C. as maximum temperature, and dependent on the technology chosen.